1. Field of the Invention
This invention relates primarily to machines such as pistons and crossheads in reciprocating engines, compressors and pumps but is applicable to any machine employing a reciprocating sliding motion.
2. Review of the Prior Art
Referring to the drawings:
FIG. 1 is a diagrammatic side elevation of a prior art reciprocating machine including a piston or crosshead, and
FIG. 2 is a diagrammatic view of the prior art piston or crosshead of FIG. 1 inclined relative to an associated guide surface to illustrate a convergent wedge-shaped film.
The machine of FIG. 1 has a rotating crank 1 and connecting rod 2 are linked by a pivot 3 to a piston or crosshead 4 which reciprocates in a cylinder or guide 5. The force 6 along the line of action of the piston combines with the connecting rod force 7 to produce a resultant lateral force 8. The sliding speed of the piston or crosshead 4 varies approximately sinusoidally from zero at each end of the stroke, where the direction reverses, to a maximum value near the mid point. The lateral force 8 also varies cyclically, its magnitude and direction depending upon the particular type of machine. For example, in most internal combustion engines now manufactured, the lateral force on the piston reverses at the end of the stroke causing the piston to cross over from one side of the cylinder to the opposite side.
As a result of long practical experience the specific loading of guide surfaces on conventional pistons and crossheads is very low compared with that on the rotating bearings, commonly one-tenth or less.
Specific loading is defined as the applied force divided by the area over which it is distributed, projected in the direction of the force-that is ##EQU1##
To obtain a low specific loading relatively large surfaces are required and these cause a concomitant frictional drag due to the shearing of the film of lubricating oil between the surfaces. Any reduction in this friction will increase the mechanical efficiency of the machine and, in the case of an engine, will reduce the fuel consumption for a given power output.
The theory and practice of lubrication confirm that parallel sliding surfaces have a limited specific load capacity and tend to also have limitations to the sliding velocity which may safely be applied and this applies to the guide surfaces already described.
Both specific loading and sliding speed capacities can be greatly increased if the surfaces are inclined very slightly so that a converging film of lubricant is formed. FIG. 2 illustrates this convergent, or wedge-shaped film 9a essental to high performance, where a surface 9 slides over a guide surface 11 in the direction of arrow 10. For optimum performance the angle 12 between the surfaces will generally be between 0.0001 and 0.01 radian.
Thus, if the sliding surface of component 4 were to be inclined relative to the non-operating surface, the lubrication would be improved, specific loading could be increased allowing a reduction in area and thereby reducing the frictional drag. An improvement in mechanical efficiency would ensue. Two inclinations would be required, one for each direction of sliding, and the no-operating surface would still contribute to frictional drag, but there could still be a net gain over conventional designs. However, a fixed inclination is only suitable for a narrow range of speed and specific load conditions whereas the piston or crosshead 4 can in many cases be subject to widely varying speed and load conditions, each cyclical in nature but not always in step.